1. Field of the Invention
The present invention relates to semiconductor processing equipment and, more particularly, to a controllable restrictor shield, located between a process chamber and a pump, having a controllably variable effective throughput area.
2. Description of the Background Art
To produce a sufficient vacuum for processing semiconductor wafers in a process chamber of a semiconductor wafer processing system, a cryogenic pump (commonly referred to as a cryo-pump) is used to attain the vacuum. Systems that utilize cryo-pumps include physical vapor deposition (PVD) systems that require an ultra high vacuum (UHV), approximately 5.times.10.sup.9 Torr, to achieve optimal process conditions and process performance.
Typically, the cryo-pump is connected to the process chamber via a conduit, where the connection point of the conduit to the process chamber is known as the "cryo-port". The area of the cryo-port opening is critical to achieving particular process results during a deposition process. Although the cryo-port has a fixed area, it is typically fitted with a cryo-pump restrictor shield (referred to herein as a cryo-shield) that defines the effective throughput area of the cryo-port. A typical cryo-shield is an aluminum plate having approximately a six inch (15.25 cm) diameter. The plate contains a plurality of apertures of predefined diameter. The cumulative area of the apertures defines the effective throughput area of the cryo-port. As such, different plates having differing effective areas are installed over the cryo-port depending upon the particular process results desired. One example of a PVD system that uses such a cryo-shield is the Endura model manufactured by Applied Materials, Inc. of Santa Clara, Calif.
Typically, before a process chamber is used to process semiconductor wafers, the chamber goes through a process known as "bakeout", where the chamber is heated by lamps to desorb and evaporate any volatile particles within the chamber. The volatile particles are removed from the chamber by pumping, e.g., usually by using a combination of pump types including turbo-pumps, cryo-pumps, and the like. Once the volatile particles have been pumped from the chamber, the chamber is allowed to cool to a nominal temperature over a period of time known as the cooldown period. A chamber is considered "qualified", for processing when the chamber achieves a sufficient vacuum (e.g., 8.times.10.sup.9 to 5.times.10.sup.9 Torr) after both bakeout and cooldown are complete. The bakeout and cooldown period can be as long as 80 hours. Thus, for a substantial amount of time, the processing equipment is being initialized and is not processing wafers.
The duration of the bakeout and cooldown period is directly proportional to the effective throughput area of the cryo-port. To facilitate a high gas flow rate during bakeout and reduce the bakeout period, it is desirable to have as large of a cryo-port effective throughput area as possible. However, the cryo-shield is in place during bakeout in anticipation of processing once the chamber is qualified. As such, the cryo-port effective throughput area is inherently limited to the aperture area of the cryo-shield. To remove the cryo-shield and increase the throughput area would require opening the chamber to atmosphere, removing the cryo-shield and then performing another bakeout cycle. The time that the system is unproductive while the throughput area is changed would be intolerable.
Additionally, chamber process pressures frequently need adjusting and fine tuning during the processing of semiconductor wafers. Since traditional restrictor shields are fixed and the effective throughput area of the cryo-port is also fixed, the processing pressures cannot be adjusted or controlled by the traditional restrictor shields.
Therefore, there is a need in the art for a restrictor shield having a controllably variable effective throughput area such that the effective throughput area of the cryo-port can be controllably increased or decreased during bakeout or processing to precisely control chamber vacuum conductance.